There has been proposed a magnetic recording and reproducing apparatus which is provided with a heading coded AC signal information read device. In such a heading device, a coded AC signal is recorded at each head position of programs on the magnetic tape. The coded AC signal consists of a combination of binary values in which a bit "1" is represented by one cycle of an AC signal of low frequency range while a bit "0" is represented by an absence of the AC signal. The coded AC signal is read out by detecting either of the polarities of the AC signal. However, following the bit "1", the reproduced output tends to include an undesired component at the position corresponding to the non-recorded portion of the bit "0" due to a phase delay of harmonic in the course of recording and reproducing. FIGS. 1a and 1b show two types of coded AC signals in which reference numerals a designate the undesired component of the reproduced outputs. As noted from these figures, the undesired component is of positive polarity when the AC signal component which is recorded or written on the bit "1" begins at the positive polarity, while the undesired component is of negative polarity when the AC signal component begins at the negative polarity. Thus, it will be noted that the undesired component is of the same polarity as that of the first half-cycle of the AC signal. In the prior device, since the level of the AC signal is detected at either of the polarities of the AC signal, the undesired component a may be read out as a bit "1". Thus, the information is disadvantageously read out in an erroneous manner.